Recurrence of persistent virus infections causes major health problems for AIDS patients and others who are severely immunosuppressed. This is particularly true for the herpesvirus family, several members of which are present in a latent form in the majority of the population. The two human gammaherpesviruses, the Epstein- Barr virus and Human Herpesvirus-8 both cause malignancies in the immunosuppressed, including non- Hodgkin's lymphoma, central nervous system lymphomas, Kaposi's sarcoma and primary effusion lymphomas. However in healthy patients, the T cell response controls latent gammaherpesvirus infection very efficiently. Currently it is not fully understood what components of the T cell response need to be lacking in order for gammaherpesviruses to break through and cause disease. Nor is it known whether immune therapies can be developed which can restore immune surveillance and prevent virus-associated disease. In this application we study murine gammaherpesvirus (MHV-68) infection, an accepted and valuable small animal model for gammaherpesvirus pathogenesis and immunology. While normal mice control latent infection with MHV-68, spontaneous virus reactivation occurs in mice that are deficient in CD4 T cells. Virus reactivation in the absence of CD4 T cells therefore provides a model similar to recurrent gammaherpesvirus- associated disease that occurs in AIDS patients, who also lack CD4 T cells. Our research, and that of other labs, has shown that MHV-68-specific CD8 T cells are still present in the absence of CD4 T cells, and are ostensibly functional, but nevertheless unable to contain virus replication. In the preliminary data for this application we describe a factor that exerts a negative influence on the antiviral T cell response, thereby allowing virus reactivation to occur. In Specific Aim 1 we determine the mechanism responsible for this effect, and how its blockade can partly restore control over virus reactivation. Our preliminary data also describe an immunotherapy that can enhance the CD8 T cell response to overcome this negative influence, very efficiently restoring viral control, and the mechanism of action of this therapy is elucidated in Specific Aim 2. Given the success of the two immunotherapeutic approaches described in this proposal, Specific Aim 3 tests whether these two therapies can be improved by combination with a therapeutic vaccine or by co-administration of both therapies. This Specific aim also describes the use of a newly developed model for gammaherpesvirus- associated tumor development, and tests the effectiveness of these immunotherapies in this model. In conclusion, there is a pressing need for novel therapies that can restore control of gammaherpesvirus infection in the immune suppressed. At present there are very limited treatment options, and those that are available, such as adoptive immunotherapy, are very expensive and difficult to perform outside of specialist centers. There is a good likelihood that therapies similar to those we describe will be effective in humans, and our studies provide essential information concerning their method of action.